The formation of underground impermeable barriers using slurry trench techniques have been widely developed in recent years and in a number off instances, attempts to utilize impervious plastic or rubber sheets to form impermeable barriers in such cut-off walls has been suggested, as for example, see Ranney U.S. Pat. No. 2,048,710, Zakiewicz U.S. Pat. No. 3,603,099, Piccagli U.S. Pat. No. 4,193,716 and Carron et al. U.S. Pat. No. 3,759,044. In Ressi application Ser. No. 252,676 filed Apr. 9, 1981, and assigned to the assignee hereof, a plastic sheath or envelope is provided in which permeable or porous material is in the sheath or envelope. Drainage nets sandwiched between two high density polyethylene films or geomembranes have been used in landfill, pollution control and other drainage systems. Typical drainage nets are dimensionally stable grids consisting of two sets of parallel strands with the intersecting strands forming overlayed sets of continuous drain channels to provide high flow capacity. Panelization of plastic films for insertion into vertical cut-off walls is disclosed in the above reference Cavalli application wherein a fluid barrier is constructed using pairs of spaced-apart plastic channel members with a plastic sheet sealingly joined to mutually facing surfaces thereof with slots formed in the oppositely facing surfaces for telescopically receiving rigid end pieces of intermediate plastic panel members and the sheath or double sheets of said Ressi application is also disclosed. In the above references Ressi application (also owned by the assignee hereof), the fill between the sheath is a porous drain material and self-starting pumps are embedded in perforated drain in the sheath for removing pollutants that may penetrate. As noted in the Ressi application, the pollutant barrier comprises first the excavation wall into which the bentonite has penetrated, a cake of bentonite formed on the excavation wall, a layer of high density polyethylene plastic, then the next layer of high density polyethylene plastic on the opposite side thereof and then a cake of bentonite and the earth wall that has been penetrated by the bentonite.
According to the present invention, a pollution barrier comprises a pair of substantially rectangular sheets which are sealingly joined along three edges to form a sheath or envelope and into which is inserted a drain net of the type manufactured and sold by the Tensar Corporation under the trademark Tensar. Rigid end members which, in the preferred embodiment, is perforated hollow pipe, are in the lateral ends of the envelope or sheath to rigidify the ends. The sealing of the lateral ends of the sheath can be by way of sealing to the perforated pipe. The drain net, which like all the plastic structures referred to in this specification, is preferably high density polyethylene which is substantially resistant to attach by a wide variety of chemicals.
In forming the excavation, a plurality of bore holes are spaced apart typically on 33 to 40 foot centers along the line of the pollution control barrier. A plastic coupling member, which for all intermediate bore holes has oppositely facing locking keyway slots formed therein, is inserted into the bore hole with a stainless steel blocking member, filling the slot so that upon positioning the coupling member in the bore hole and verticalizing same, the space around the coupling member is filled with a excavatable cementious material. After the excavatable cementious material has set, the earth in the space between bore holes is excavated as by a clam shell, backhoe excavator or kelly rig excavator, etc. so as to remove the soil and earth in the space therebetween all the while maintaining the excavation full of bentonite or other slurry trenching liquid material. Bentonite is preferred because it forms a mud cake on the excavation wall which serves as a barrier to pollutant flow. After the intervening soil has been excavated between a given pair of coupling elements, the steel protection members are withdrawn from the slots so as to open the keyway slots and then the plastic panel members described above are inserted with the end perforated drain pipe members fitting telescopically within the keyway slots of the coupling member. Then the space between the sidewalls and the plastic panel members is filled with a cement bentonite mixture preferrably by the tremie pipe method and the displaced bentonite is used in adjacent excavations or in forming the cement-bentonite mixtures.
The basic object of the invention is to provide an improved process and construction of underground pollution control barriers.